The present invention relates to a method and apparatus for the automatic charging and supervision of an accumulator battery which, as a source of stand-by voltage is connected to the outlet of a controllable rectifier arrangement that feeds a subsequent load, e.g. a telephone exchange.
Accumulator batteries are practically always present in current supply systems for such tele-communication equipments which demand a high degree of safety and uninterrupted DC-supply. In present systems, the battery is normally fully charged and does not participate in the delivery of energy to the tele-communication equipment except for short duration transient loads.
The task of the battery is to supply the tele-communication equipment during a voltage break down with necessary energy without interruption and during the time of the break down or until a possible stand-by element has started. Electronic devices in tele-communication equipments do not allow high values on transient voltages. The battery, which has very low impedance then proves itself to be an economical as well as an appropriate means to reduce such transients. The low impedance of the battery is also employed as to suppress disturbing voltages from rectifiers or other disturbing devices in tele-communication equipments in order to reduce crosstalk.
As mentioned above and to keep the battery fully charged, a charging device is required which charges the battery to full charge with no risk of overcharge and which is independent of such battery parameters as temperature and age.
In order to keep the battery fully charged when simultaneously supplying the tele-communication equipment with necessary current, it is necessary to adapt the outlet voltage to the battery type and to the number of series connected cells. The lead-battery, which is the most common battery type, requires a voltage of 2.15-2.25 V/cell. The battery manufacturer usually prescribes a value between these limits, e.g. 2.22 V. This value should be contained with an accuracy of .+-.0.5%-.+-.1% so that the best operation condition for the battery will be guaranteed and thereby obtain a long lifetime.
When charging the battery, it is of importance that 100% charging degree can be achieved, since there is a risk of sulphation of uncharged elements in the battery-cells for insufficient charge of the battery, whereas for excess charge there is a risk for an accelerated corrosion, increased water consumption and, in unfavorable cases, such a high and rapid rise in temperature that the battery is destroyed.
One example of a previously known charging method consists in measuring the period of discharge and, after that, charging the battery for a period which is dependent on the period of the discharge. This method is not sufficiently accurate for obtaining 100% charging degree, since no attention is paid to the number of discharged ampere hours or to the power loss that may vary.
Another known method consists in measuring the number of discharged ampere hours and, after that, charging the battery with an equivalent number of ampere hours. Also this method is not sufficiently accurate since, like the first mentioned method, it pays no attention to the battery losses.
Still another known method is based upon measurement of the charging current which is delivered to a battery at a determined cell voltage. At a cell voltage of, e.g. 2.35 V/cell, the battery is fully charged if the charging current is less than 4 mA/Ah and the charging can be interrupted. The drawback with this method is, however, that the final value of the charging current is dependent on the temperature and age of the battery.